Small-mouthed, thin-walled metal containers are utilized for many purposes, most predominantly for commercial distribution of food and beverages. These containers are commonly called "tin cans". Although it is not a widely known fact, almost all "tin cans" used for food and beverage purposes are coated on the interior with a thin, i.e., a thickness less than about one mil, comestible (food grade) resinous coating. Millions of pounds of resin are utilized each year in coatings for food and beverage containers. These organic resinous coatings, which may vary in composition dependent upon the ingredient which they will contact, are necessary to prevent contamination of the food or beverage by the metal container, whether said container is tin plated steel or aluminum. Contamination of food and beverage by the metal container generally affects flavor, occasionally makes the food or beverage unwholesome, and frequently affects appearance. Also, the coating promotes the shelf life of the "canned" ingredients.
A number of different resinous compositions have been utilized with success as coatings for metallic containers, particularly food and beverage containers. Although water-based coatings have been available for a number of years, the films or coatings resulting therefrom have not been satisfactory for food and beverage containers; for example, beer develops unacceptable taste when stored for normal durations in metal containers coated with a water-based resinous coating.
The mainstay of the can coating industry has been organic, solvent-based coatings--in spite of the fact that the solvent, which evaporates upon application of the coating, is 80% of the weight of the material and often has a greater cost than the resin which remains on the container. Organic, solvent-based coatings have been successful, however, because thin coatings can be applied to metal containers which do not affect substantially the taste of the food or beverage. Solvent-based coatings, however, have a very distinct disadvantage--a very large quantity of solvent evaporates into the atmosphere adjacent container coating facilities. These organic solvents are generally noxious and frequently toxic.
One approach towards eliminating solvents from container coatings is to use 100% solids coating systems; e.g., the liquid styrene-polyester system, epoxy resins and the like. Liquid systems containing 100% coatings solids; i.e., everything in the liquid system reacts or interacts to become an integral part of the resinous (polymeric) coating formed upon a substrate, have severe limitations in that few polymeric systems lend themselves to a liquid system wherein one of the reactants is dissolved in another reactant. Also, those 100% solids liquid systems available have such high viscosities that applications by spray techniques is impracticable, if not impossible, especially when thin films are desired. A further limitation of 100% solids coatings for containers is the inclusion in the coating of a certain quantity of monomer or low molecular weight polymer which, even when present only as parts per million, produces odor and usually contributes taste to the coating.
Another type of 100% solids coating material is a powdered, resinous material. "Powder coatings", as the term is commonly used, have been applied to objects primarily by fluid bed and electrostatic spray techniques. Fluid bed techniques are unsatisfactory to coat food and beverage containers since such techniques coat both the interior and exterior of the can, thereby consuming an uneconomical amount of resin. Also, coatings formed in a fluid bed tend to be relatively thick; e.g., five mils and greater. Electrostatic spray techinques work very well for most objects, although coatings less than one mil thick are difficult to obtain.
However, electrostatic spray principles (a charge on the powder and an opposite charge on the object to be coated) do not work when the interior of a small cylindrical metal container is to be coated. An effect known as the "Faraday Cage Effect" occurs when powder containing an electrostatic space charge is propelled towards the interior of an oppositely charged metal cylindrical container having one end thereon, resulting in formation of a partial coating.
A further impediment to replacement of solvent-based coatings for containers, aside from the difficulties encountered in making very thin, pore-free films, has been the coating speed required by industry. Commercial container lines, particularly beverage container lines, move at a speed of 250 to 300 containers per minute.